I don't understand that at all. What's the exception? The rule seems pretty cut and dry
"No Club or club shall Play, or list as a named substitute, any Player or player who, in the same Season, has already played in a Match by entering the field of play whether in the starting eleven or by being used as a substitute by another Club or club in the Competition."
ETA
Never mind, just seen the rule change mentioned on the St mirren thread